Construction platforms are widely used in the construction of reinforced concrete or steel framed buildings wherein the outer skin of the building is not load bearing, and is not put in place until after the main supporting structure of the building has been finished, and its major internal fittings have been installed.
Such construction platforms customarily comprise an inboard portion, which rests upon and is fixed to an edge margin of a building floor, and an outboard portion, including a landing deck, which extends as a cantilever from the inboard portion beyond the edge of the floor. Conveniently, the inboard portion may comprise a base frame adapted to rest on the floor and a plurality of extendible props rising from the base frame to the underside of the next higher floor whereby the base frame, and therefore the platform as a whole, is clamped in position.
Hitherto, construction platforms have fallen into two classes, namely fixed deck platforms and movable deck platforms. Fixed deck platforms have the inboard and outboard portions integrally united as a single structure. They are simple in design, robust and inexpensive compared to movable deck platforms. However they suffer from the disability that they project from the building during the whole of the construction period and require to be staggered in the vertical direction across a face (or faces) of the building so that higher platforms do not obstruct the rope of a crane depositing a load onto, or lifting a load from, a lower platform. This, in turn, requires the use of an expensive long reach crane, to service all of the platforms at the site, if the crane operates from a fixed location, as is usual.
Movable deck platforms, examples of which are shown in U.S. Pat. No. 4,444,289 (Jungman) and International Patent Application No. PCT/AU94/00509 (Preston) have the outboard portion movably mounted on the inboard portion, so that it may be retracted when not in use to leave the face of the building free of obstructions. This overcomes the mentioned disability of fixed deck platforms, but at the expense of a much more complicated and heavily built platform because of the need to provide a two part, telescoping base frame with sufficient overlap between the parts to enable the bending moment applied by the extended outboard part to the inboard part to be resisted. Further more the outboard part has to be heavily designed to give it appropriate rigidity as it derives little or no bracing effect from being clamped to the building floor. For these reasons, movable deck platforms have not been widely adopted by comparison with fixed deck platforms.
The present invention arose from the simple appreciation that a crane rope must extend through the center of gravity of the load, and is thus necessarily spaced from the face of a building when lifting or lowering a load beside the building. Thus a degree of permanent projection of a higher platform is not objectionable, in that it will not interfere with the deposition of a load onto a lower platform even though it be directly underneath the higher platform.